The development of secondary motions due to the nonzero second normal stress difference is observed and analyzed by tracking the motion of the interface between two batches of the same low density polyethylene, each with a different pigmentation. in this article we focus on straight and tapered channels with a square cross section. Three-dimensional numerical simulations are also performed on the basis of a multimode differential viscoelastic fluid model. These calculations allow the investigation of the development of secondary motions in space. Predictions are compared with the experiments: good agreement is found between the experimental observations and their numerical counterparts. The magnitude and the relevance of secondary motions are also discussed. A numerical analysis is carried out on the sensitivity of secondary motions with respect to the nonlinear properties of the viscoelastic model. Finally, we make a brief attempt to explain the mechanism which drives the secondary motions.